The present invention relates to inductive loops for traffic detection. The invention is also concerned with traffic monitoring apparatus comprising at least one of such inductive loops for detecting and potentially classifying traffic passing over the loop.
Inductive loops for traffic detection are well known and used commonly for monitoring traffic flow along the lanes of roadways. Typically, a loop may comprise a rectangular outline loop of conductor buried just beneath the surface of the roadway and connected to energising and detecting equipment at the side of the roadway. The loop is energised with alternating current at a selected frequency to produce a corresponding alternating magnetic field in the space above the loop. Vehicles passing over the loop affect the inductance of the loop which can be detected by the detection equipment. Typical prior art loops comprise a single rectangular winding having a length, in the distance of travel of vehicles along the roadway lane, which may be a substantial proportion of the length of vehicles travelling along the roadway, say 1 meter or more, and a width transversely of the direction of travel only slightly less than the width of the roadway lane. The detection signal produced in such inductive loops responds to the metal mass of a vehicle passing over the loop, particularly the engine and drive train, and also chassis components of longer vehicles. For detection of vehicles as a whole, loops are designed to ensure a good detection signal is achieved as the vehicle passes by. U.S. Pat. No. 3,983,531 discloses a typical inductive loop sensor roadway installation of this kind.
There is also a requirement to count the number of axles of vehicles passing along a roadway, so that multi axle vehicles for example can be distinguished from ordinary domestic automobiles for example. Accordingly, loops have been designed which are intended to be specifically sensitive to the axles, or more particularly to the wheels, of vehicles passing over the loop. U.S. Pat. No. 5,614,894 discloses a wide variety of inductive loops used for the detection of the wheels of vehicles passing along the roadway. A separate loop may be used for each wheel track in each lane of the roadway and the patent indicates that the overall length of the loops in the direction of traffic movement should be relatively short, comparable to the footprint on the roadway of the vehicle wheels to be detected by the loops. The patent suggests a length in the traffic direction of 15 cms for loops intended to detect the wheels of domestic automobiles, and 30 cms for loops intended for detecting the wheels of trucks.
It is an object of the present invention to provide a further inductive loop sensor design, specifically adapted for the detection of the wheels of vehicles passing over the loop, which can have a general purpose application to all kinds of vehicles using the roadway.
Accordingly the present invention provides an inductive loop sensor for detecting vehicles travelling along the lane of a roadway. The sensor comprises a continuous conductive loop configured to have a central conducting segment and outer conducting segments spaced on opposite sides of the central segment. An electric current in the loop flows in a first transverse direction along the central segment and in a second transverse direction opposite to the first transverse direction along each of the outer segments. The loop is aligned on the roadway lane so that the central and outer segments are transverse to the traffic flow direction in the lane. The distance between the outer segments of the loop are selected to be not greater than about 60 cms.
The resulting loop can provide, when energised, a magnetic field which extends above the roadway by no more than about 30 cms. In this way, the sensor can be made relatively less sensitive to the passage over the loop of the main bulk of vehicles, particularly engine, drive train and large chassis members. By comparison, the loop will respond specifically to metal components of the wheels of the vehicle travelling on or just above the roadway surface. In particular the loop will respond to the steel bracing in steel braced tyres, or alteratively to the metal of the wheel and wheel hub itself.
Importantly, a single size of loop can be used for detecting both domestic automobile wheels and also the wheels of large trucks.
The loop could be wide enough to cover an entire lane but preferably has a width across the traffic flow direction of between about 100 and about 140 cms. Conveniently, the width of the loop across the traffic flow is less than that which would allow the wheels at both ends of an axle of a vehicle to be detected simultaneously by the loop. A width of loop of about 120 cms is considered appropriate.
Preferably the distance between the outer segments of the loop is about 45 cms. This then gives good discrimination between the effect of a wheel and the influence of the engine/drive train/chassis of a vehicle which passes over the loop just outside the influence of the magnetic field.
The central segment of a loop should be symmetrically located between the outer segments.
The loop may be configured as a figure-of-eight, or as a pair of multiturn windings of opposite hand connected in series.
The invention also provides traffic monitoring apparatus comprising at least a first inductive loop sensor, a generator to energise this first loop sensor with a detection signal and a detector responsive to changes in the detection signal in the loop sensor to provide an indication of a vehicle crossing the loop sensor. The loop sensor comprises a continuous conductive loop as described above.
In a further embodiment the apparatus comprises a second inductive loop sensor having the same form as the first sensor, where the first and second sensors are aligned spaced apart one after the other along the roadway lane in the traffic flow direction. The generator energises both the first and second sensors with respective detection signals and the detector is responsive to changes in these signals in each of the sensors to provide an indication of the direction of travel.
The loop sensors may have similar dimensions and neighbouring outer segments of the two loop sensors are typically spaced apart in the traffic flow direction by between 15 and about 25 cms.
The invention still further contemplates a method of counting the number of axles of vehicles travelling along a lane of a roadway, in which,
at least one inductive loop sensor in the form of a continuous conductive loop as described above is installed on or in a surface of the roadway lane. This sensor is energised with a detection signal to generate a magnetic field which extends above the surface of the roadway lane by not more than about 30 cms. Changes in the detection signal corresponding to the passage of vehicle wheels over the loop are detected.